Fossils found at the Sterkfontein Caves in South Africa reveal nearly four million years of hominin and environmental evolution. Since research began at the site in 1936 with the discovery, by Robert Broom, of the first adult hominin of the genus Australopithecus, it has become famous for the hundreds of Australopithecus fossils yielded from excavations of ancient cave infills, including iconic specimens such as the cranium known as Mrs. Ples and the Little Foot skeleton.

The majority of Sterkfontein's wealth of Australopithecus fossils has been excavated from an ancient cave infill called 'Member 4' - the richest deposit of Australopithecus fossils in the world. Over the last 56 years of Wits-led research at Sterkfontein, the age of Member 4 at Sterkfontein have remained contested, with age estimates ranging from as young as about 2 million years ago, younger than the appearance of our genus Homo, back to about 3 million years.

New research presented in a paper published in the journal PNAS re-evaluates the age of Australopithecus from Member 4 at Sterkfontein together with the Jacovec Cavern, which contains a few additional hominin fossils in a deeper chamber in the cave.

"The new ages range from 3.4-3.6 million years for Member 4, indicating that the Sterkfontein hominins were contemporaries of other early Australopithecus species, like Australopithecus afarensis, in east Africa," says Professor Dominic Stratford, director of research at the caves, and one of the authors on the paper.

The new ages are based on the radioactive decay of the rare isotopes aluminum-26 and beryllium-10 in the mineral quartz. "These radioactive isotopes, known as cosmogenic nuclides, are produced by high-energy cosmic ray reactions near the ground surface, and their radioactive decay dates when the rocks were buried in the cave when they fell in the entrance together with the fossils," says Professor Darryl Granger of Purdue University in the United States and lead author on the paper.

Previous dating of Member 4 has been based on dating calcite flowstone deposits found within the cave fill, but careful observations show that the flowstone is actually younger than the cave fill and so it underestimates the age of the fossils.

"This re-assessment of the age of Sterkfontein Member 4 Australopithecus fossils has important implications for the role of South Africa on the hominin evolution stage. Younger hominins, including Paranthropus and our genus Homoappear between about 2.8 and 2 million years ago. Based on previously suggested dates, the South African Australopithecus species were too young to be their ancestors, so it has been considered more likely that Homo and Paranthropus evolved in East Africa," says Stratford.

The new dates show that Australopithecus existed at Sterkfontein almost a million years prior to the appearance of Paranthropus and Homo, providing more time for them to evolve here, in the Cradle of Humankind, and placing the hominins from this site front and center in the history early human evolution.

"This important new dating work pushes the age of some of the most interesting fossils in human evolution research, and one of South Africa's most iconic fossils, Mrs Ples, back a million years to a time when, in east Africa, we find other iconic early hominins like Lucy," says Stratford.

"The redating of the Australopithecus-bearing infills at the Sterkfontein Caves will undoubtably re-ignite the debate over the diverse characteristics of Australopithecus at Sterkfontein, and whether there could have been South African ancestors to later hominins," says Granger.

Research Report:Cosmogenic nuclide dating of Australopithecus at Sterkfontein, South Africa

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